Numerous references teach using mixed matrix membranes which comprise a continuous phase polymer carrier with porous particles dispersed therein. Examples include U.S. Pat. No. 4,925,459 to Rojey et al. and U.S. Pat. No. 5,127,925 to Kulprathipanja et al. The membranes are particularly useful for separating gases from a mixture or feedstream containing at least two gas components, generally of differing effective diameters.
Membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching one component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity is defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desirable for membrane materials to have a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
Under the proper conditions, the addition of porous particles may increase the relative effective permeability of a desirable gas component through the polymeric membrane (and/or decrease effective permeability of the other gas components), and thereby enhance the gas separation (selectivity) of the polymeric membrane material. If the selectivity is significantly improved, i.e., on the order of 10% or more, by incorporating porous particles into a continuous phase polymer, the mixed matrix membrane may be described as exhibiting a “mixed matrix effect”. A selectivity enhancement test will be described in detail below.
This “mixed matrix membrane” concept is described in publications such as U.S. Pat. Nos. 6,503,295; 6,562,110; and 6,508,860 and U.S. Patent Publication Nos. 2002/0056369 and 2002/0053284—using porous, molecular-sieving entities to enhance separation performance. The so-called “mixed matrix effect” relies on the principle that inclusion of highly size- and shape-selective molecular sieves (such as zeolites or carbon molecular sieves) having pore dimensions that can discriminate penetrants within a polymeric matrix may substantially improve the overall selectivity of the hybrid membrane. Such enhanced selectivity may be much higher than the selectivity achievable using the neat polymer as a membrane alone.
A significant problem with using such mixed matrix membranes is the selectivity-productivity trade-off of membranes. This trade-off is encountered when optimizing membranes for maximum selectivity and maximum productivity. Generally, these two properties operate counter to each other. In other words, higher selectivity membranes generally have lower productivities while lower selectivity membranes generally offer higher productivities.
Ideally, a mixed matrix membrane will have a high permeability. This will allow membranes with a minimal amount of surface area to treat to separate a large volume of mixed gases. Economically, a more expensive polymer and/or porous particle can be used in a smaller size membrane as compared to a much larger membrane made of less expensive neat polymer or made using less expensive polymer/sieve particles which are significantly less productive. Still, the membrane should not suffer a significant loss in selectivity relative to using a membrane made of only the neat polymer.
The present invention addresses shortcomings in previous mixed matrix membranes which have sacrificed membrane productivity (permeability) in order to achieve higher levels of selectively.